How do I generate random numbers to fill in empty rows?

You can do this using rand(),checksum(), andnewid()`:

update t
    set exam = rand(checksum(newid())) * 101,
        assignment = rand(checksum(newid())) * 101;

In SQL Server, rand() is (essentially) called once for a query. To get around this, you can give it a new seed. You could also use other columns in each row as the seed value, if you wanted reproducible results.